When separation angles and fabric clash
By Rick Davis
Moiré has plagued screen printers since the introduction of the halftone. At one time, it was considered an unavoidable flaw of halftone printing. Today, however, printers are less tolerant of this unwanted patterning effect, and controlling moiré has become a science for many of those who print process color. Garment screen printers are challenged by moiré more than any other segment of the industry. Their problems stem from the multistep recreation of an image in screen printing, which involves the interplay of a number of grid-like patterns at various stages of the process. In other words, the printer begins with film separations consisting of halftone dot patterns. The film is then used to expose the stencil on the screen, which itself is a grid-like pattern of mesh. Finally, the image is printed onto a garment, which has a knitted surface with a regular stitch pattern. Controlling the way these patterns combine during printing is essential for avoiding moiré. The situations described above introduce the three types of moiré you're likely to encounter: primary, secondary, and tertiary moiré. 1. Primary moiré This form of moiré occurs when two halftones (separations of the same image) at different angles are combined during printing, and the respective angles create an undesirable interference pattern (Figure 1). 2. Secondary moiré This moiré is seen when halftone angles and/or line counts clash with the weave of the screen mesh to produce a patterning effect. 3. Tertiary moiré The final type of moiré occurs when a halftone is printed onto the garment, and the knitted surface of the fabric interferes with the halftone dots, creating unwanted patterning (Figure 2). Situations that will lead to primary and secondary moiré are relatively easy to identify. To check for primary moiré, simply align your film positives together in different combinations of pairs before you use them to expose screens. If the halftone angles are not ideal for any given pair, a moiré pattern will become readily apparent when you superimpose one film over the other. Similarly, you may be able to spot potential secondary moiré as soon as you expose a screen if the halftone clashes with the mesh. Obviously, it would be best if you could spot potential secondary moiré before you go to press. But in some cases, the moiré won't become apparent until you actually begin printing with the screen. The main difference between primary or secondary moiré and tertiary moiré is that primary and secondary moiré are rooted in your screens. This means that if the moiré is present when you expose the screens (either because you used the wrong halftone angles or because of mesh interference), you'll see an identical moiré pattern in each of the final prints you create with those screens. Tertiary moiré, on the other hand, does not occur consistently on each print you generate (Figure 3). This is because of variations in garment knit that occur from shirt to shirt, even when they're made of the same fabric. Additionally, this form of moiré strikes because the location of the knitted pattern relative to the screens shifts each time you load a new garment onto the press. The following factors influence whether a given garment/image combination will lead to tertiary moiré: • garment characteristics, including the fabric's yarn count (yarn thickness) and stitch density (the number of stitches per inch at which the garment was knitted) • artwork characteristics, including halftone line counts (lines of halftone dots per inch) and screen angles The garment Few printers consider the construction or quality of the fabric they're using before they run a process-color job on garments. The fact is, of all the variables that can lead to moiré, none is more influential than fabric construction. A fabric's surface characteristics are what determines your ability to reproduce artwork on a particular garment. The most important elements that affect surface construction are the gauge of the yarn (known as the yarn count) and the stitch density of the knit (measured in stitches per in.). Together, these elements influence the size of wales (rows of vertical stitches) on the fabric. If the wales are too large and loosely spaced relative to your halftone dots, you'll get tertiary moiré (see Figures 4A and 4B ). Many garment printers think that yarn count refers directly to the thickness of the yarn. While comparing yarn-count values will let you determine whether one yarn is thicker than another, the value doesn't refer to the measured thickness of the yarn. Instead, it refers to the number of 840-yard lengths of the yarn it takes to equal one pound. So a 24-singles yarn is one that requires 24 840-yard lengths to equal one pound, and an 18-singles yarn would need 18 840-yard lengths to yield one pound. In other words, the lower the yarn count, the thicker the yarn. To reduce the potential for tertiary moiré, try to select fabrics knitted with a low count yarn (thicker yarn) and a high stitch density. This combination of features provides the smoothest, most consistent surface on which to reproduce halftone patterns. Although the garment will still have wales, they'll be smaller and packed tightly together, which means they'll be less likely to interfere with the halftones. A thinner yarn, lower stitch count, or both effectively reduces the fabric mass of the garment. In practical terms, this means the fabric provides less surface area on which to print. Consequently, the chance of halftone dots falling between wales or yarn fibers increases, and your ability to accurately reproduce the halftone diminishes. Although you might be tempted to look for T-shirts that feature both thin yarns and high stitch counts, let me save you the trouble. This combination of features, which is usually found in dress shirts, does provide a smooth, consistent fabric surface. However, the thinner yarns would make T-shirts feel stiff and have poor drapability. This is why you won't find T-shirts with these fabric characteristics. The artwork The decisions you make when creating artwork for garment screen printing can also increase or decrease the potential for tertiary moiré. Even if you use a quality garment constructed with thick, tightly knitted yarn, you should try to avoid the following elements in your artwork: • large flesh-tone areas and other image elements that consist primarily of highlight dots or combinations of highlight and quarter-tone dots • large areas of uniform color, such as the image shown in Figure 2 • areas with overlaid halftone gradations that fade off to 0% Any designs that incorporate such elements are capable of contributing to tertiary moiré. So if a customer supplies artwork with these characteristics, make sure to run test prints and warn the customer about the potential for moiré. Line count A related artwork aspect to consider is the line count of your halftone. As a general rule of thumb, the higher the halftone line count relative to the garment's stitch count, the greater the likelihood of tertiary moiré. Essentially, higher line counts mean smaller halftone dots, which are more likely to fall between wales and fabric fibers than larger dots (Figures 5A and 5B). Halftone angles To reduce the potential for primary and secondary moiré, many textile screen printers working with process color use halftone screen angles offset by 15° or 30° from each other. A typical angle set is cyan = 105°, magenta = 75°, yellow = 90°, and black = 45°. In some cases, the cyan and magenta angles may be switched. Additionally, while positioning the yellow at 90° may lead to moiré from mesh interference, the light color prevents it from being easily noticeable. Still, some shops like to reduce all potential for moiré and run yellow at 60° instead. Tertiary moiré occurs when the angles of the magenta, cyan, or black separations clash with the wales of the fabric (Figure 6A and 6B). Because variables such as halftone line count and fabric stitch density affect the potential for tertiary moiré, you can't predict what combination of halftone angles and wale positions will cause it. However, you can use your film positives to experiment. Position each of your film positives (for C, M, or K) independently over the desired image location on the garment at the angle you wish to print it. Tertiary moiré patterns should be immediately apparent if the angle is unsuitable. You can then rotate the positive and estimate a new angle that is less likely to result in moiré. But make sure to adjust the angles for the other halftones so that they continue to be offset from one another by 15° or 30°; otherwise, the risk of primary moiré increases. If no moiré pattern is visible, you still need to rotate the positive slightly in both directions to see if moiré appears. This will allow you to anticipate moiré that could occur due to variances in garment placement when shirts are loaded onto the press. Also be aware that in some cases, tertiary moiré results from the combined interaction of two or more overprinted separations and the fabric surface. This problem is the most difficult to predict without generating test prints. To reduce the chance of any form of moiré and avoid the issue of separation angles altogether, you might consider using stochastic separations rather than conventional halftones. However, while stochastic halftones can reduce the potential for primary and secondary moiré due to their random dot arrangements, their effectiveness in controlling tertiary moiré is more limited. Stitch pattern locations can shift from shirt to shirt, and stochastic halftones may still occasionally combine with the knit to form unwanted artifacts in the printed image. But this occurs infrequently, so stochastic separations may be a good alternative, particularly if you're printing images with large highlight areas on thin, loosely knitted garments. Once again, you'll need to generate test prints of the image to be sure. The screens The quality of your screens also affects your ability to control and eliminate tertiary moiré. This means you need to select screens with a thread count appropriate to support the image resolution you desire. And always maintain high screen tension and make sure tension levels are consistent on all the screens for a particular job. Otherwise, you risk dot gain during printing, which can enhance tertiary moiré effects that would not have appeared with properly tensioned screens. Conclusion If you find that your prints are exhibiting tertiary moiré, the first and most simple option for minimizing the effect is to select a new type of garment that has a higher stitch density. Unfortunately, you don't always have the option of choosing the fabric on which you print. The next option is to decrease the line count of your halftones to produce bigger dots that are more likely to land on the surface of the shirt, rather than between wales or fabric fibers. Finally, if all else fails, you can adjust the angles of your separations until the problem is cleared up or until you determine that the fabric is simply too coarse to accommodate the image. Tertiary moiré is a significant obstacle in printing garments with process color. But by understanding and correcting the situations that most often lead to tertiary moiré, you can minimize or completely avoid the problem. About the author Rick Davis is a 23-year veteran of textile screen printing. He has an extensive background in production and has worked as a printing process troubleshooter for both Wilflex Inks and Hanes. Rick is a frequent speaker at screen-printing trade events and a regular contributor to industry publications.
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